1. Field of the Invention
The present invention relates to apparatus for guiding magnetic tape. In particular, the invention relates to a tape guide mechanism for dynamic tracking control in the presence of transient tracking errors.
2. Description Relative to the Prior Art
In the following description of the relevant prior art, reference is made to FIG. 1, of the accompanying drawings, which is an elevation view partially in section, of a tape guide mechanism known in the art.
With reference to the magnetic tape recording art, tracking is the process of keeping a playback head on the path of a track already recorded on magnetic tape. The purpose of tracking control is to adjust the position of the playback head relative to the record track or vice versa, so that the playback head is aligned with the record track for maximum signal-to-noise output.
To facilitate tracking, tape guides, which serve to keep the tape laterally straight, are normally strategically placed, for example, at the entrance and exit point of the record/playback heads. One particularly preferred type of tape guide includes means for producing a slight lateral force in one direction, for maintaining a given edge of an advancing tape in continuous contact with a fixed reference surface, without damaging the tape edge or buckling the tape.
U.S. Pat. No. 4,403,720, which is assigned to the assignee of the present invention, discloses a tape guide of this general type. The basic components of such a tape guide 10, which is shown in FIG. 1 of the drawings, are as follows. The tape guide 10 has a post 12 for supporting a hollow tape guide roller 14. The post 12 is shown in FIG. 1 as attached to and extending vertically from a baseplate 16. The post 12 may extend at any other angle and may, if desired, have the form of, or be replaced by, a rotatable shaft or similar device.
The tape guide 10 includes a sleeve 18 surrounding the post 12 inside the guide roller 14. The sleeve 18 has an inside diameter that is larger than the outside diameter of the post 12, for receiving an O-ring 24 between the post and the sleeve. The O-ring 24, which is seated in a peripheral groove 26 on the post 12 and a corresponding groove on the sleeve 18, serves as a fulcrum to provide tilting motion of the sleeve relative to the plane 28 of the O-ring.
Ball bearings 30, which are arranged in races 32 on opposing sides of the plane 28 of the O-ring 24, couple the guide roller 14 to the sleeve 18 so that the guide roller is rotatable about the sleeve. Thus, the spaced bearings 30 mount the tape guide roller 14 for rotation about the post 12 and tilting motion with the sleeve 18.
A tape guide flange 34, which has a reference surface 36 extending radially from the post 12, is precisely positioned on the post 12, adjacent one end of the tape guide roller 14, to control the lateral position of tape (not shown). In practice, the reference surface 36 is a layer or stratum of ceramic or other hard material, and may be rotatable or non-rotatable.
While the tape guide 10 is generally applicable to guiding magnetic tape through a recorder, it is particularly well adapted for controlling the lateral position of the tape as it advances past a head stack consisting of a plurality of closely spaced record/playback heads. In this configuration, tape guide 10 is positioned immediately before the head stack so that an advancing tape wraps partially around the tape guide roller 14. The tape guide 10 is positioned so that the reference surface 36 is parallel to the tape transport direction, with the centerline, denoted 38, of the advancing tape being offset a distance, denoted d, with respect to the plane 28 of the O-ring 26, in the direction of the guide flange 34.
The tape approaches the tape guide 10 tangential to the surface of the roller 14. If the edge of the tape is not in contact with the guide flange 34, the roller 14 tilts with the sleeve 18, because of the offset of the centerline 38 of the tape from the O-ring 26. In response to this tilting movement, the tape moves axially in the direction of the flange 34. When the edge of the tape engages the reference surface 36, the tape transport direction becomes parallel to the surface 36 again.
In practice, the tape guide 10 of FIG. 1 maintains an edge of the tape generally perpendicular to and in continuous contact with the reference surface 36 during tape transport movement. The forces acting on the tape from the fulcrum effect of the O-ring 26 and its offset are balanced with the forces acting on the tape from the guide flange 34.
U.S. Pat. No. 4,150,773 discloses an alternative arrangement for guiding a tape with an edge thereof in continuous contact with a reference surface. To decrease forces acting on the edge of the tape, and thus prevent damage to the tape, a rotatable tape-contacting surface has a plurality of spaced, parallel rings of rubber, the rings being inclined toward a tape guide flange. In response to tape advancing movement, the rings in contact with the tape deflect. Because of the coefficient of friction of tape on the rings, the tape moves with the rings toward the flange, whereby an edge of the tape is maintained generally perpendicular to and in continuous contact with a stationary reference surface of the flange. The rings of the rotatable surface return to their nominal radial position when not engaged by the advancing tape.
There are a variety of standard magnetic tape recording formats currently used commercially. For example, one standard tape recording format presently used in the United States, includes 14 parallel record tracks. With a 1-inch wide tape, each track has a track pitch of 70 mils (50 mils record track width and 20 mils guardband width). Tape guides of the type disclosed in the above-mentioned patents provide acceptable tracking control for this recording format and other formats in which the track pitch is of comparable size.
A general problem arises however when prior art tape guides are used for tracking control of information that is recorded on even more closely spaced record tracks. For example, other magnetic tape recording systems use one or more recording heads which sweep transversely across a magnetic tape at high speed, as the tape is moving. With some of these systems, a track pitch as small as 1.2 mils (1.0 mil record track width and 0.2 mil guardband width) may be used. Thus, even a small deviation in alignment of the tape as it passes over a playback head, compared to passing over a record head, may either result in significant loss of playback signal strength, or playback of the wrong track.
A further problem arises when playback of narrow-width record tracks occurs on apparatus that is different than the apparatus that was used for recording. In this situation, precision tracking control has been found to be even more necessary because of both static and dynamic sources of tracking error including mechanical tolerance build-up inherent in different playback apparatus, variations in the scanning surface and the scanning plane of the playback head, variations in track centerline locations, and other secondary considerations such as changes in tape tension because of temperature and humidity variations.
With the various tape guides known in the prior art, tracking control of narrow-width record tracks may be achieved effectively, at least in theory, when a tracking error source is static, or when the error source is only slowly varying with time, such as when the dimensions of the tape change with humidity and temperature. This is because a static or bias error can be compensated during playback by operator adjustment of the playback apparatus.
However, when a tracking error source exists which is changing more rapidly and is thereby transient, such as when one phase of a scanning surface during playback is different from the corresponding phase of the scanning surface during recording, mistracking results that can not be corrected effectively by operator adjustment. When the record tracks are of a relatively narrow width, significant loss of playback signal may occur. Accordingly, prior art tape guides suffer from a disadvantage in that they are not suitable for tracking control of narrow-width record tracks in the presence of transient tracking errors.